Write line identifying Catch2 test interface in --help output of test builds

[graydon]

This adds code to print a single identifying prefix line Catch2 v2.13.10 when you run stellar-core --help on a BUILD_TESTS build.

This, along with a small amount of configuration, is enough to enable vscode and this common C++ testing extension to do automatic unit test discovery and integration with vscode's test system. This gives you:

• A searchable list of tests in the sidebar
• One-click access from that list jump to a test, run a test, or (most importantly) run the debugger on a test
• A little button in the source code at the test header to do the same
• Log capture and display of each test run and its results

The configuration required in .vscode/settings.json is just the following:

{
    "testMate.cpp.test.advancedExecutables": [
        {
            "pattern": "${workspaceFolder}/src/stellar-core",
            "catch2": {
                "prependTestListingArgs": ["test"],
                "prependTestRunningArgs": ["test"],
                "ignoreTestEnumerationStdErr": true,
                "testGrouping": {
                    "groupBySource": {}
                }
            }
        }
    ]
}

You then get this splendid experience: